RF step attenuators are an important part of many general purpose electronic instruments such as spectrum analyzers, network analyzers, S-parameter test sets, signal generators, sweep generators, and high frequency oscilloscopes, just to name a few. Special purpose test sets, such as those used to test wireless communications equipment are also important users of RF step attenuators. Decades ago an RF step attenuator was a manually operated device: the human hand generally turned a knob. With the advent of automated test systems under computer control, and the more recent advent of automatic test equipment that has its own internal processor, has a sophisticated repertoire of testing abilities, and has extensive instrument-to-instrument communication abilities, the need for attenuators that are electrically controlled has steadily grown, and continues to do so. The increases in performance, both in accuracy and in higher frequencies of operation, have placed additional demands upon the nature of the desired attenuators. Furthermore, stand-alone instrument grade programmable (solenoid operated) step attenuators usable in the microwave region are simply too big and too costly for many of today""s designs, where much of the circuitry is integrated.
One prior art response to this situation is represented by the A150 line of ultra-miniature attenuator relays from Teledyne (www.teledynerelays.comxe2x80x9412525 Daphne Avenue, Hawthorne, Calif., 90250). They are small, approximately three-eighths by seven-sixteenths of an inch in length and width by less than a third of an inch in height. They are usable to 3 GHz, have an internal matched thin film attenuator (pad) available in Pi, T or L sections, and are available in a variety of attenuations of from 1 dB to 20 dB. This family of relays provides the xe2x80x9cstepxe2x80x9d in attenuation by replacing the pad with a length of conductor. The mechanical arrangement for doing this is set out in U.S. Pat. No. 5,315,723, issued May 24, 1994 and entitled ATTENUATOR RELAY. It does not appear that the length of conductor that replaces the pad is a section of genuine controlled impedance transmission line.
FIG. 1 is a generalized representation of a prior art step attenuator relay 1, such as the A150 attenuator relay. An RF input 2 is coupled to the moving pole of a SPDT switch 4, and an RF output 3 is taken from the moving pole of a SPDT switch 5. Switches 4 and 5 are operated together by the solenoid of the relay (not shown), with the effect that either an attenuator section 6 or a conductor 7 is connected between the RF input 2 and the RF output 3. It is not so much that this arrangement is defective, it works up to some upper frequency where geometry begins to significantly influence circuit behavior. At higher frequencies the stray coupling capacitances 10 and 11 (which are around one hundred femto farads) allow conductor 7 to begin to shunt the attenuator 6, and RF currents will flow around the attenuator 6, driven by the voltage drop across the attenuator itself. There are minor stray reactances within the conductor 7, which we have indicated in a very general way by the series inductances 8 and the shunt capacitance 9. At higher frequencies the stray coupling capacitances 10 and 11 combine with the stray reactances 8 and 9 to form a resonant circuit that poisons the attenuation inserted by the relay 1. In the case of the A150 this happens at around 4 GHz.
Recent developments have occurred in the field of very small switches having liquid moving metal-to-metal contacts and that are operated by an electrical impulse. That is, they are actually small latching relays that individually are SPST or SPDT, but which can be combined to form other switching topologies, such as DPDT. (Henceforth we shall, as is becoming customary, refer to such a switch as a Liquid Metal Micro Switch, or LIMMS.) With reference to FIGS. 2-5, we shall briefly sketch the general idea behind one class of these devices. Having done that, we shall advance to the topic that is most of interest to us, which is a technique for fabricating on a hybrid substrate a high performance high frequency step attenuator using a collection of such relays.
Refer now to FIG. 2A, which is a top sectional view of certain elements to be arranged within a cover block 2 of suitable material, such as glass. The cover block 2 has within it a closed-ended channel 18 in which there are two small movable distended droplets (23, 24) of a conductive liquid metal, such as mercury. The channel 18 is relatively small, and appears to the droplets of mercury to be a capillary, so that surface tension plays a large part in determining the behavior of the mercury. One of the droplets is long, and shorts across two adjacent electrical contacts extending into the channel, while the other droplet is short, touching only one electrical contact. There are also two cavities 16 and 17, within which are respective heaters 14 and 15, each of which is surrounded by a respective captive atmosphere (21, 22) of an inert gas, such as CO2. Cavity 16 is coupled to the channel 18 by a small passage 19, opening into the channel 18 at a location about one third or one fourth the length of the channel from its end. A similar passage 20 likewise connects cavity 17 to the opposite end of the channel. The idea is that a temperature rise from one of the heaters causes the gas surrounding that heater to expand, which splits and moves a portion of the long mercury droplet, forcing the detached portion to join the short droplet. This forms a complementary physical configuration (or mirror image), with the large droplet now at the other end of the channel. This, in turn, toggles which two of the three electrical contacts are shorted together. After the change the heater is allowed to cool, but surface tension keeps the mercury droplets in their new places until the other heater heats up and drives a portion of the new long droplet back the other way. Since all this is quite small, it can all happen rather quickly; say, on the order of milliseconds.
To continue, then, refer now to FIG. 1B, which is a sectional side view of FIG. 1A, taken A through the middle of the heaters 14 and 15. New elements in this view are the bottom substrate 13, which may be of a suitable ceramic material, such as that commonly used in the manufacturing of hybrid circuits having thin film, thick film or silicon die components. A layer 25 of sealing adhesive bonds the cover block 12 to the substrate 13, which also makes the cavities 16 and 17, passages 19 and 20, and the channel 18, all gas tight (and also mercury proof, as well!). Layer 25 may be of a material called CYTOP (a registered trademark of Ashai Glass Co., and available from Bellex International Corp., of Wilmington, Del.). Also newly visible are vias 26-29 which, besides being gas tight, pass through the substrate 13 to afford electrical connections to the ends of the heaters 14 and 15. So, by applying a voltage between vias 26 and 27, heater 14 can be made to become very hot very quickly. That in turn, causes the region of gas 21 to expand through passage 19 and begin to force long mercury droplet 23 to separate, as is shown in FIG. 3. At this time, and also before heater 14 began to heat, long mercury droplet 23 physically bridges and electrically connects contact vias 30 and 31, after the fashion shown in FIG. 2C. Contact via 32 is at this time in physical and electrical contact with the small mercury droplet 24, but because of the gap between droplets 23 and 24, is not electrically connected to via 31.
Refer now to FIG. 4A, and observe that the separation into two parts of what used to be long mercury droplet 23 has been accomplished by the heated gas 21, and that the right-hand portion (and major part of) the separated mercury has joined what used to be smaller droplet 24. Now droplet 24 is the larger droplet, and droplet 23 is the smaller. Referring to FIG. 4B, note that it is now contact vias 31 and 32 that are physically bridged by the mercury, and thus electrically connected to each other, while contact via 30 is now electrically isolated.
The LIMMS technique described above has a number of interesting characteristics, some of which we shall mention in passing. They make good latching relays, since surface tension holds the mercury droplets in place. They operate in all attitudes, and are reasonably resistant to shock. Their power consumption is modest, and they are small (less than a tenth of an inch on a side and perhaps only twenty or thirty thousandths of an inch high). They have decent isolation, are reasonably fast with minimal as contact bounce. There are versions where a piezo-electrical element accomplishes the volume change, rather than a heated and expanding gas. There are also certain refinements that are sometime thought useful, such as bulges or constrictions in the channel or the passages. Those interested in such refinements are referred to the Patent literature, as there is ongoing work in those areas. See, for example, the incorporated U.S. Pat. No. 6,323,447 B1.
To sum up our brief survey of the starting point in LIMMS technology that is presently of interest to us, refer now to FIG. 5. There is shown an exploded view of a slightly different arrangement of the parts, although the operation is just as described in connection with FIGS. 2-4. In particular note that in this arrangement the heaters (14, 15) and their cavities (16, 17) are each on opposite sides of the channel 18. A new element to note in FIG. 5 is the presence of contact electrodes 91, 92 and 93. These are thin depositions of metal that are electrically connected to the vias (30, 31 and 32, respectively) and serve to ensure good ohmic contact with the droplets of liquid metal. The droplets of liquid metal are not shown in the figure.
It would be desirable if we could take advantage of the small size and otherwise desirable characteristics of the LIMMS relays to provide an instrument grade attenuator relay usable to up to, say, eight or ten Gigahertz. What to do?
A solution to the problem of resonance within an attenuator relay caused by stray coupling capacitances to, and stray reactance within the switched conductor that replaces the attenuator section, is to ensure that the stray coupling capacitances are diminished to as low a value as possible, and to ensure that the conductor is a section of controlled impedance transmission line that matches the system into which the attenuator relay has been placed. A substrate having SPDT LIMMS switches on either side of a switched transmission line segment and its associated attenuator, all of which are fabricated on the substrate, will have significantly lower stray coupling capacitance across the open parts of the switches when the attenuator segment is in use. This will increase the frequency for the onset of the resonance driven by the RF voltage drop across the attenuator. A reduction in the amplitude of the resonance can be obtained by including on the substrate an additional pair of SPST or SPDT LIMMS damping switches at each end of the transmission line segment. These damping switches each connect a terminating resistor to the ends of the transmission line segment when the attenuator section is in use. This loads the resonator and reduces the amplitude of the resonance. Still further improvement can be obtained by locating one of the damping switches and its termination resistor near (but preferably not exactly at) the middle of the transmission line segment.